Q. A star moves away from earth at speed $0.8\ c$ while emitting light of frequency $6 \times 10^{14}Hz$. What frequency will be observed on the earth (in units of $10^{14}Hz$) ($c =$ speed of light)
A) 0.24
B) 1.2
C) 30
D) 3.3
Explanation
(b) Observed frequency $v^{'} = v\left( 1 - \frac{v}{c} \right)$
$\Rightarrow v^{'} = 6 \times 10^{14}\left( 1 - \frac{0.8c}{c} \right) = 1.2 \times 10^{14}Hz$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.